The Natural History of Atlantic Salmon
Much of the following information was taken directly from a chapter dealing with Atlantic salmon (Stolte 1986) in the National Audubon Society's Wildlife Report of 1986.
The Atlantic salmon lives as an adult in the sea but spawns in freshwater rivers and small streams. After the eggs hatch, the young salmon remain in freshwater for one or more years, then descend to the sea to feed and grow for at least a year before returning to freshwater to reproduce. After breeding they return to the sea. While in the sea, Atlantic salmon are silvery on their sides, silvery white underneath, and brown, green, or blue on their backs. For a short time after they enter rivers and streams to spawn, salmon remain quite silvery and often referred to as "fresh and bright fish." They gradually lose the silvery color and become darker, taking on a bronze and brown coloration as spawning approaches.
The Atlantic salmon's range extends from Portugal to the Arctic Circle in the eastern Atlantic, includes Iceland and southern Greenland, and encompasses the Ungava region of northern Quebec southward to the Connecticut River of New England in the western Atlantic. Salmon from both the eastern and western Atlantic live in the feeding grounds off Greenland. Salmon of the eastern Atlantic also feed in the Baltic, in areas near the Faroe Islands, and elsewhere in the eastern Atlantic. Food items principally consist of fishes such as herring, capelin, and sand eels. Large zooplankton such as euphausiids and amphipods also are important in the salmon diet. Salmon are themselves preyed upon by seals, sharks, pollack, tuna, skates, halibut, cod, striped bass, bluefish, and other predators. Man, too, is a significant predator.
Anadromy and Homing
One of the most intriguing of all aspects of the salmon's life history is its homing instinct. The fact that salmon return to their parent stream has been known for several hundred years. The importance of stream odors in the orientation of fish also has been well demonstrated. Research indicates that, in salmon, the smell of the parent stream is imprinted during a short period before the young descend to the sea. Thus, as maturing salmon approach the coastal areas from the open ocean, they probably locate parent streams by smell. How the salmon navigate in the ocean, far from their parent streams, still remains a mystery.
Atlantic salmon may leave the sea to spawn in their parent stream during any month of the year. In New England, adult Atlantic salmon enter their parent streams from May through October with May, June, and July being the most important time periods. After entering freshwater, adult salmon cease to feed. They will not eat again until they re-enter the sea some six months to a year later.
Returning salmon usually are between three and six years old, but individuals up to 11 have been reported. In New England returning salmon range in age from two to at least six years, with four being the predominate age - two years in freshwater and two years (winters) in the ocean. Salmon that return after one year (one-sea-winter) at sea are called grilse and will usually weigh between two and six pounds. Those returning after two years (winters), often called multi-sea winter salmon (MSW) or two-sea-winter (2SW) salmon, weigh between six and 15 pounds. Those returning after three years (winters) at sea, often called MSW salmon or three-sea-winter (3SW) salmon, may weigh more than 20 pounds. Of course, older individuals and repeat spawners (salmon that have spawned in (a) previous year(s)) may weigh even more. Throughout its range, the Atlantic salmon spawns during the fall and early winter months. Spawning typically occurs in late October through November in New England.
The female chooses the nest site, usually a gravel-bottom riffle area above or below a pool, and with her caudal fin excavates a pit into which eggs are deposited. More than one male usually participates with a single female in fertilizing the eggs. This process is repeated again and again until all or nearly all of the female's eggs have been deposited. The series of pits into which the eggs have been deposited and covered with gravel is called a "redd." The female deposits roughly 700 eggs per pound of body weight, from 2,000 to 15,000 eggs. The adult fish after spawning are called "kelts" and may return to the sea immediately or during the following spring, as is typical in New England. Kelts that return to the sea in the spring are often called black salmon or slinks. Only a small percentage of the kelts, primarily females, will reach the sea and return in later years as repeat spawners.
Eggs normally hatch in late March and April, depending on water temperature. Water temperatures below 50 degrees F are desirable for normal egg development, and temperatures in the low 40s are considered optimum. The sac fry or "alevins", as the newly hatched salmon are called, remain buried in the gravel until the yolk sac has been absorbed. Actual emergence of the alevins, now called "fry", from the gravel occurs from March through June. The fry disperse from the redd site and rapidly assume the coloring of the life stage referred at as "parr". These young salmon have eight to 11 narrow, vertical, pigmented bands on their sides (parr marks) with a single red spot between each band. In New England, the young salmon (after hatching from the eggs) spend between one and three years in freshwater, the norm being two years.
During their freshwater residency, young salmon are opportunistic feeders, preying on the most abundant food items. Aquatic insect larvae and nymphs (chironomids, mayflies, caddis flies, black flies, and stone flies) are the principal food items. However, terrestrial insects are eaten and probably are an important part of the diet during certain periods of the year. Young salmon are also prey to a number of predators, including kingfishers, American mergansers, eels, various trout species, pike, and pickerel.
At a size of five to eight inches, the parr undergo physiological and morphometric transformations that prepare them for migration to the sea and the transition from a stream- bottom animal to a pelagic ocean fish. This transformation is known as "smoltification", and in the migratory stage, which normally occurs during the spring, the parr are more properly called "smolts". After entering the sea the smolts, now referred to as "post-smolts", migrate to the distant oceanic feeding grounds. Salmon originating from New England rivers will be found in the Greenland waters and along the Labrador and Newfoundland coasts.
The size of Atlantic salmon populations, especially in New England rivers, is governed to a large degree by the quality, quantity, and accessibility of the spawning and nursery habitats. Good spawning habitat includes beds of stones measuring one-half to four inches in diameter. These gravel beds promote the movement of clean, well-oxygenated water through the redd, which is critical since salmon eggs may be deposited as deep as 12 inches. Spawning habitat should be well dispersed throughout the nursery habitat.
Salmon nursery habitat typically is composed of shallow riffle areas interspersed with deeper riffles and pools. The substrate pebbles, ranging from one-half to greater than nine inches in diameter, afford adequate cover for the juvenile salmon. Clean, well-oxygenated water is a necessity. The young salmon also require relatively warm water for growth. They grow very slowly at temperatures below 45 degrees F and experience optimal growth in streams with daily peaks of 72 to 77 degrees F.
Returning adult salmon must have access to the spawning grounds. An open, unobstructed river is ideal. Where obstructions, such as impassable dams, occur, fish passage facilities must be provided. The distance traveled upriver may range from 10 to 600 miles. Once in the river, adult salmon making long migrations require refuge from the swift current and will periodically stop and lie in resting pools. Upon nearing the spawning grounds adult salmon will take up residence in holding pools. Holding pools have the cover, depth, temperature regime, and water velocities preferred by the adults.
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